1. Field of the Invention
This invention relates to radar systems and more particularly to coherent side lobe cancellation sensing and control functions.
2. Description of the Prior Art
A side lobe suppression system has been described by D. M. Jahn, Antenna Side-lobe Suppression System, U.S. Pat. No. 3,094,695. Jahn teaches a system in which both a directional antenna and an omni-directional antenna are provided, each having an independent receiving system channel. The gain of the omni-directional antenna-receiver combination is provided to be substantially equal to the principal side lobe signal gain of the directional antenna-receiver combination. Thus the magnitude of a signal received in the principal side lobe of the directional antenna will be substantially equal to the magnitude of that same signal as received in the omni-directional antenna. However, a signal received in the main beam of the directional antenna will be considerably larger than either of the aforementioned signals. In Jahn these signals are coupled to a first gate such that when the signal received in the omni-directional antenna is equal in magnitude to the signal received in the directional antenna, that is, there is no signal in the main beam of the directional antenna, a signal called a blanking signal is applied to a second gate. This blanking signal operates to block the presentation of signals from the directional antenna upon a display device. These two signals, coupled to said first gate, are called a blanking signal as derived from said omni-directional antenna channel and an enabling signal as derived from said directional antenna channel. It should be noted that the blanking signal applied to the first gate is not identical with the blanking signal applied to the second gate, however, this is the nomenclature as used in Jahn.
When the signal as received in the directional antenna is considerably larger than the signal as received in the omni-directional antenna, indicating the presence of a signal in the main beam of the directional antenna, a third signal is produced proportional to the difference in the magnitudes of the signals. This third signal, when applied to said first gate, disables it and prevents the formation of a blanking signal to said second gate thus allowing presentation of the display of a signal received in the main beam of the directional antenna. Thus signals received in the side lobes but not in the main beam of a directional antenna pattern are eliminated while signals received in the main beam are displayed.
A side lobe cancellation system has also been described by T. W. Howells, Intermediate Frequency Side Lobe Canceller, U.S. Pat. No. 3,202,990. It is to be noted that Jahn, supra, used two separate receiving channels, one for the directional antenna and another for the omni-directional antenna and utilized a difference in received signal magnitude in order to accomplish elimination of jamming at the video level. Howells points out that the disadvantage of such an approach is that it requires matching two complete receivers in band pass, time delay and amplitude response. In addition, all intermediate frequency signal processing equipment used with the radar must be completely duplicated if video side lobe cancellation is to be accomplished. Howells, therefore, as indicated by his title, seeks to accomplish side lobe cancellation without matching two complete receivers, by performing such cancellation as a function of intermediate frequencies. In Howells, advantage is taken of the phenomenon that the form of jamming signals received by the two antennas differs only by a gain term and a phase shift caused by the path length difference experienced by the wave front due to the physical separation of the two antennas. In order to effect cancellation of the jamming signal, the signal from the omni-directional antenna is heterodyned by means of a first local oscillator down to a first intermediate frequency and the signal from the directional antenna is heterodyned by means of a second local oscillator down to a second and different intermediate frequency.
These two intermediate frequencies are again heterodyned and narrow band filtered resulting in the production of yet another signal whose frequency is equal to the difference between said first and second intermediate frequencies. The amplitude of this resulting signal is proportional to the average power of the jammer and has the phase of the jamming signal as received at the omni-directional antenna. This signal is again heterodyned with the jamming signal which produces a signal now at the radar intermediate frequency that is phase equalized and with an amplitude proportional to the average power of the jammer. This last signal is introduced to the radar intermediate frequency channel through a subtraction circuit in such a way that the residual jammer signal approaches zero in the radar intermediate frequency channel. It is a feature of the teaching of Howells's invention that the system defined cancels signals which have a relatively high power, while it is relatively insensitive to target returns having a low average power. It is to be noted in connection with this feature that the power density at the radar from a target echo is proportional to the inverse fourth power of the distance between radar and target while the power density at the radar from a jammer is proportional to the inverse second power of the distance between radar and jammer.
Charles S. Gager et al have described a cancellation system using an ultra sonic delay line in Apparatus for Double Cancellation Utilizing One Delay Line in a Moving Target Indicating System, U.S. Pat. No. 3,222,602. The invention takes advantage of the fact that input signals to moving target indicator systems exhibit the characteristic that fixed targets have a constant amplitude from one interpulse period to the next, however, due to the Doppler frequency beat between return target signals from a moving target and the radar reference signal, moving targets show a variable amplitude from one interpulse period to the next. In accomplishing the double cancellation of the invention, an intermediate frequency carrier is first modulated with radar echo pulses and then passed simultaneously through a direct line and a delay line. These signals are then amplitude detected and the signal from the direct line is subtracted from the delayed signal. The resulting difference signal is then used to phase modulate the original intermediate frequency carrier. By comparing the phase between the direct and delayed carrier signals in a phase detector, a double cancellor video output may be extracted therefrom.